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Abstract

We demonstrate a novel type of Fourier Transform Spectrometer (FTS) that can be realized with CMOS compatible fabrication techniques. This FTS contains no moving components and is based on the direct detection of the interferogram generated by the interference of the evanescent fields of two co-propagating waveguide modes. The theoretical analysis indicates that this type of FTS inherently has a large bandwidth (>100 nm). The first prototype that is integrated on a Si3N4 waveguide platform is demonstrated and has an extremely small size (0.1 mm2). We introduce the operation principle and report on the preliminary experiments. The results show a moderately high resolution (6 nm) which is in good agreement with the theoretical prediction.

Figures (6)

Fig. 2 (a) Simulation results of the interference pattern between the two waveguides in the case of single wavelength injection. The position where one should position the grating is marked by the blue frame. (b) A zoom in of the interferogram in the grating region. (c) A zoom-in of the first several periods of the intensity oscillation. [9]

Fig. 4 (a) The setup used in the preliminary experiments, where we use a lensed fiber (LF) to couple the signal into the chip, and project the interferogram onto a CCD camera with an objective lens (OBJ). Two laser sources (L1 and L2) with the polarization controller (PC) can be applied simultaneously or individually. The CCD camera and OBJ (both shown inside the blue dash box) can move together to scan the whole interferogram. (b) The entire interferogram reconstructed by stitching of 8 snapshots. (c) One of the snapshots obtained during the scanning.

Fig. 5 (a) Plot of the intensity profile extracted from the measured interferogram. The inset shows the computed sinusoidal interferogram with same period and total length as the measured one: the red curve has the same power decay rate while the green curve has no decay. (b) The spectrum calculated from the interferogram measured as shown in (a). (c) A zoom-in of the spectrum displayed in (b) together with the green and red spectrum resulting from the Fourier transform of the calculated interferogram in the inset of (a).

Fig. 6 Two sets of experimental results obtained with two laser sources. The interferogram (a) and the spectrum (b) of the input laser light of 900 nm & 822 nm. The interferogram (c) and the spectrum (d) for 900 nm & 876 nm laser light. The insets show the snapshots taken by the CCD camera.